A service area in a mobile communication system is composed of cells, each of which is covered by a base station. Cells are classified into a macro cell, a micro cell, and a pico cell according to the size of the cell. Typically, the macro cell has a large coverage, where transmission power of a base station is high. On the other hand, the pico cell has a small coverage, where transmission power of a base station is low. For example, micro cells and pico cells are installed in sites where traffic flows heavily.
FIG. 1 shows a propagation environment in which two base stations BS1 and BS2 have different transmission power. Downlink transmission power from the base station BS1 to a mobile station decreases according to the distance between the base station BS1 and the mobile station. Similarly, downlink transmission power from the base station BS2 to the mobile station decreases according to the distance between the base station BS2 and the mobile station. At the point A in FIG. 1, downlink transmission power from the base station BS1 is equal to downlink transmission power from the base station BS2. Between the point of the base station BS1 and the point A, downlink transmission power from the base station BS1 is higher, and thus the base station BS1 is suitable for downlink communications. Between the point of the base station BS2 and the point A, downlink transmission power from the base station BS2 is higher, and thus the base station BS2 is suitable for downlink communications. Typically, transmission power of the base station BS1 providing the macro cell is higher than transmission power of the base station BS2 providing the pico cell, the distance between the base station BS1 and the point A is longer than the distance between the base station BS2 and the point A.
On the other hand, pass loss increases according to the distance from the base station. In other words, the inverse of path loss decreases according to the distance from the base station. At the point B in FIG. 1, the inverse of path loss associated with the base station BS1 is equal to the inverse of path loss associated with the base station BS2. Uplink signal quality from the mobile station to the base station depends on the inverse of path loss. Thus, between the point of the base station BS1 and the point B, uplink signal quality to the base station BS1 is higher, and thus the base station BS1 is suitable for uplink communications. Between the point of the base station BS2 and the point B, uplink signal quality to the base station BS2 is higher, and thus the base station BS2 is suitable for uplink communications. Since path loss depends on the distance from the base station, the point B is approximately at the midpoint between the base station BS1 and the base station BS2.